Lift systems for lifting objects are currently known in the art which use pairs of transmission chains, i.e., so-called engagement chains, that move up and down by integrally engaging from each other, such as the system shown in Japanese Patent Publication No. 3370928.
In one such system, shown in FIGS. 6 and 7, inner and outer plates 510 and 540 of the engagement chain 500 have buckling restricting flat surfaces 512 and 542, which are formed so as to support a load by facing each other when the inner plates 510 and the outer plates 540 are engaged and integrated by sprockets.
As shown in FIGS. 6 and 7, the buckling restricting flat surface 542a extends along an imaginary line L, with a vertical pitch of 2P and an angle of α from horizontal that runs from a center point PU of a pin press-fitting hole 541c to center point PD of a pin press-fitting hole 541d. 
As shown in FIG. 6, when a downward load F is applied to the chain, slippage may occur between the outer plates 540a and 540a due to the component force F1 of the load F acting along the buckling restricting flat surface 542a along the imaginary line L. This slippage may cause a gap between a connecting pin and a bush and a gap between hook portions.
Thus, because there is relative slippage between the outer plates 540, the prior art pair of engagement chain 500 has problems in that it is prone to be inclined or buckled when engaged with an opposing portion of the chain, thus lowering its buckling strength.
FIG. 7 illustrates another problem with the engagement chain 500 which takes place when an upward force F′ acts on the outer plate 540a through the buckling restricting flat surface 542a. Specifically, a component force F1′ of the force F′ is lost in the tangential direction of the buckling restricting flat surface 542a, so that an excessive driving force is required to drive and lift the engagement chain 500.